Combine Use of NVR & Back Pressure Valve

That's where you would put the back pressure valve. It's there to provide a positive pressure to pump against, without it metering may not be accurate. Eg in an extreme case, where the supply tank is at higher level than the dosing point, without a back pressure valve liquid can flow even if the pump is not running.

You might also need a pressure relief valve, unless the pump has another way of preventing overpressure. Pressure relief valve can be same type of valve as back pressure valve (but set to a higher pressure). Difference is it's installed on a branch from the pump discharge line, and relieves to atmosphere, or to the supply tank, or at a pinch to the pump suction pipe.

thanks codemaster.